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Turtles Natural History

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LIFE HISTORY

Females of all species of turtle (except perhaps Chelodina rugosa, see below)leave the water to deposit their eggs in cavities or depressions constructed in thesurrounding sand, soil or litter. Turtles abandon the eggs once they are laid anddo not protect the hatchlings. All species have distinct annual breeding seasons.

Synchronisation of breeding with periods of ample food availability isunnecessary, because turtles can accumulate resources over time to provide forthe demands of vitellogenesis (Chessman 1978; Georges 1983; Kuchling &DeJose 1989). This ability is carried to extremes in marine species, withChelonia mydas taking in excess of one year to prepare for a breeding season(Limpus & Nicholls 1988). Nor is there a need for breeding to coincide withconditions suitable for copulation, as the male and female cycles are notparticularly closely synchronised in chelonians. Males of most Australianspecies have sperm in the epididymides in all months, and females of somespecies overseas have been found to store sperm for several years.

There are three principal constraints on the reproductive patterns of chelonians.

Firstly, females must nest when conditions are conducive to adult activity. Theymust also nest when the conditions to follow are conducive to embryo survivaland ultimately, embryonic development. Thirdly, they must ensure that thehatchlings emerge when conditions are conducive to their survival. Knowledgeof each of these constraints in the context of the ecology of a species is generallysufficient to explain its reproductive strategy.

A plethora of solutions for meeting these constraints has evolved. There isvariation among species and plasticity among populations of the same species inthe timing of egg laying, clutch size, clutch frequency and incubation period.Effective incubation may be prolonged by cold torpor arrest during the wintermonths, embryonic diapause, delayed hatching, embryonic aestivation, anddelayed emergence from the nest after hatching (Ewert 1985).

Reproductive Patterns for Freshwater Turtles

Two broad reproductive patterns can be identified among Australian freshwaterturtles—one temperate and one tropical—though whether there is an overridingtaxonomic influence on these patterns is debatable (Legler 1985; Kuchling1988).

Winter provides a major interruption to adult activity and growth, and animpediment to embryonic development, for turtles of the temperate zones.

 Emydura krefftii from Fraser Island in Queensland exhibits a typical

reproductive pattern for temperate-zone turtles of both the northern and southernhemispheres (Georges 1983). Mating occurs all year round with peaks in theautumn and spring. Sperm are present in the epididymides of males all yearround, but spermatogenesis is post-nuptial with a peak in testicular activity inautumn and a cessation of testicular activity during the breeding season. Yolk begins to accumulate in the ovaries of females in late summer and continuesthrough winter, presumably by a transfer of material from fat stores to theovaries. Ovulations and nesting begin in early spring. Up to three clutches arelaid by each female between early spring and mid summer. Clutch size rangesfrom four to ten eggs, depending upon the size of the female. Hatchlings emergefrom nests in mid to late summer and make their way to the water. There isample time for incubation and hatching to occur before the onset of winter.

Although it is the most prevalent pattern, spring nesting and summer hatching isnot universal among Australian freshwater turtles, even within the temperatezones (see Chapter 21). For example, Chelodina oblonga nests in the spring,early summer and mid summer. Pseudemydura umbrina nests in the spring. In

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both species, hatching is initiated by winter rains (Burbidge 1981; Clay 1981).Chelodina expansa nests in the autumn and winter (Goode & Russell 1968;Georges 1984; Legler 1985).

Tropical freshwater species are freed from the constraints of reduced wintertemperatures, and show the greatest diversity in reproductive patterns. Rainfall

throughout the Australian tropics is markedly seasonal (Taylor & Tulloch 1985).There is typically a dramatic rise in water levels in the monsoonal wet season(December to March) and a corresponding drop in water levels in the wet-drytransitional months (April, May) and the following dry season (June to August).Freshwater species inhabiting the tropics often have only a very short period inwhich to find the relatively dry ground suitable for nesting, and they have solvedthe problem in a variety of ways. Chelodina rugosa lays eggs underwater or insaturated soils (Kennett, Christian & Pritchard 1993a). If local conditions havedried, C. novaeguinea may move to more permanent water bodies or aestivateuntil conditions are appropriate (Covacevich, Couper, McDonald & Trigger1990a; Kennett, Georges, Thomas & Georges 1992) (see Chapter 21).

Tropical Emydura spp. inhabit permanent water and, unconstrained by seasonaldisappearance of water, they nest at the same time as their southern counterparts.Their eggs incubate and hatch during the northern dry-season. Pig-nosed turtles,Carettochelys insculpta, also inhabit permanent water, and nest between lateAugust and mid November (Webb et al. 1986; Georges & Kennett 1989).

After about 65 to 70 days of incubation, and rapid development for eggs of theirsize, Carettochelys embryos are at a final stage of development and are quitecapable of hatching. Instead, they enter a form of embryonic aestivation. Theirmetabolic rate and demand for oxygen drops precipitously (Webb et al. 1986)and they wait for an appropriate stimulus before hatching. Webb et al. (1986)showed that immersion in water was sufficient to arouse the torpid offspring and

hatching followed. In the field, both flooding and torrential rain can stimulatehatching (Fig. 22.2; Georges 1987).

Figure 17.1 Egg laying in the chelid Emydura macquarii  [T. Wright]

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Clearly, hatching of the young turtles is delayed until the first heavy rains of thewet season flood the nests or saturate the surrounding sands. Presumably thehatchlings gain some benefit from this strategy, hatching into water where theremay have been none before, and the rain may have opened up new areas intowhich the hatchlings can disperse to feed and seek shelter. Hatching in responseto a discrete stimulus may ensure synchronised hatching and so help to

overcome variation in development rates caused by thermal gradients in nests(Georges 1992; Thompson 1989). Predators may be satiated as a result of simultaneous hatching of all nests on a nesting bank (Carr 1967), and thefloodwaters responsible for hatching may have dispersed potential predatorspreviously concentrated in the contracted waterbodies.

Reproductive Patterns for Marine Turtles

Marine turtle eggs require nest temperatures between 25° to 33°C and a nest in alow salinity, well-ventilated substrate with high humidity, placed where it has alow probability of being flooded or eroded (Miller 1985; Maloney, Darian-Smith, Takahashi & Limpus 1990). These conditions can be met above the storm

surge level on most tropical or subtropical beaches protected from strong waveaction by headlands or intertidal reef flats. Nesting is distinctly seasonal andrestricted to the summer months on subtropical beaches (Limpus 1971a). Closerto the equator, the period of suitable temperatures is longer and some beachesmay be suitable for successful incubation all year round (Limpus, Miller, Baker& McLachlan 1983a). The success of a rookery depends upon environmentalparameters, such as beach stability, the seasonal mosaic of nest temperatures thatdetermine hatching success and hatching sex ratio, and the proximity of offshorecurrents for dispersal of hatchlings to suitable oceanic feeding areas.

With the onset of the breeding season, adult males and females migrate fromfeeding grounds to copulate near the nesting area (Limpus, Miller, Parmenter,

Reimer, McLachlan et al. 1992; Limpus, Parmenter, Baker & Fleay 1983c;Parmenter 1983). There is no pair bond between individuals and copulation witha number of different partners during the mating season is normal (Fig. 19.3;Limpus 1993; Harry & Briscoe 1988). The female stores the sperm from her

Figure 17.2 Egg laying in the cheloniid, Chelonia mydas [T. Wright]

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several mates for use later in the breeding season. At the completion of matingthe males depart, presumably returning to the distant feeding grounds (Limpus1993). Each female moves to an area adjacent to her selected nesting beach andcommences production of eggs, fertilising them from her sperm store. Becauseof the mixture of sperm she carries, several males usually contribute to thefertilisation of any one clutch (Harry & Briscoe 1988). The female comes ashore

to nest, usually at night, several weeks after her first mating. For those beachesfronted by reef flats, nesting coincides with the higher tides.

Within the one nesting season, each female typically lays several clutches atabout two weekly intervals (Hirth 1980; Limpus 1985; Limpus, Fleay & Baker1984a; Limpus, Fleay & Guinea 1984b). Between clutches the female moves

 just offshore from the nesting beach to make the next clutch of eggs, againfertilising them from stored sperm. Breeding turtles do not feed, or else feed toonly a limited extent, while migrating, courting or making eggs at the nestingbeach area. They live on the fat reserves deposited before the breeding season.

The number of eggs and egg size varies among species.  Natator depressus laysabout 50 billiard-ball size eggs per clutch and three clutches per breeding season

(Limpus 1971a; Limpus et al. 1984a).  Eretmochelys imbricata lays about 132ping pong ball-sized eggs per clutch in about three clutches per season, whileChelonia mydas lays about 115 intermediate-sized eggs per clutch and about sixclutches per breeding season (Limpus, Parmenter, Baker & Fleay 1983d).Females lay their eggs high up on the beach usually within the vegetated strand.Miller (1985) has described the embryology of marine turtles. Eggs hatchs atabout 6–13 weeks after laying, depending on incubation temperature, asdescribed further in Chapter 19.

Females usually return to the same beach or island to lay several clutches withinthe one nesting season. However, a small percentage will lay on more than onebeach within a few hundred kilometres of the initial nesting site (Limpus et al.

1984a, 1984b). At the completion of the nesting season the female returns to thesame feeding ground that she left at the start of her breeding migration (Fig.19.3; Limpus et al. 1992). Individual females breed every 2 to 8 years, generallyreturning to nest on the same beach (Limpus 1985; Limpus et al. 1984a, 1984b).This behaviour, and the annual use of traditional nesting beaches, has led to theassumption that a marine turtle returns to nest on the precise beach of her birth.In reality, the homing is probably not so exact. Genetic studies suggest that thefemale returns to breed in the general region of her birth (Bowen, Kamezaki,Limpus, Hughes, Maylan et al. 1995; Norman, Moritz & Limpus 1994; Gyuris& Limpus 1988).

Sex Determination

Sexual differentiation of the embryo is profoundly influenced by incubationtemperature in many marine, freshwater and terrestrial turtles (reviewed by Bull1980, 1983; Ewert & Nelson 1991). For most species, females are produced athigh temperatures and males at low temperatures. A very narrow range of temperatures, referred to as the threshold or pivotal temperature, produces bothmales and females (Bull 1983). A few species have upper and lower thresholds,and females are produced at the extremes of temperature (Yntema 1976; Gutzke& Paukstis 1984). Sex determination is not temperature dependent in thoseAustralian chelids studied to date (Bull, Legler & Vogt 1985; Georges 1988a;Thompson 1988; Palmer-Allen, Beynon & Georges 1991), whereasCarettochelys insculpta (Webb et al. 1986; Georges 1992) and all marine

species (Limpus & Miller 1980; Limpus, Reed & Miller 1985) have the trait.

Eggs of Carettochelys insculpta incubated at a constant 32°C, or hotter, produce100% females whereas those incubated at 30°C or cooler produce 100% males(Webb et al. 1986). In the field, hot exposed nests produce females, cool shaded

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nests produce males, and nests intermediate in temperature produce hatchlingsof both sexes (Georges 1992). In nests that produce hatchlings of both sexes,males tend to emerge from the deepest, coolest eggs and females tend to emergefrom the uppermost, hottest eggs. The ecological significance of temperaturedependent sex determination is obscure.

Sex determination in marine species has been better studied. Females spreadtheir nesting over a series of beaches which encompass a range of nesttemperatures either side of the pivotal temperature, the theoretical temperaturethat should produce a 1:1 sex ratio (Limpus, Reed & Miller 1983b). As a result,the sex ratio for each species, or breeding stock, depends on the proportion of clutches laid in each of these warm and cool nesting areas. Cyclones, with theirassociated high rainfall and cooling effect on the beaches, can modify thegeneral seasonal trend in hatchling sex ratio from a rookery. A cyclone cancause short-term cooling events with up to 5°C drops in nest temperature over afew days. Should this happen at mid-incubation, clutches can contain all males,while clutches emerging a few days earlier or later than these may contain allfemale hatchlings (Reed 1980).

Growth and Demography

The order Chelonia includes some of the most fecund amniotes on earth.Estimates of reproductive potential range from 4.4 for the chelid, Pseudemyduraumbrina, to 690 per year for the cheloniid, Chelonia mydas. Typically, there is astrong positive relationship between clutch size and maternal body size(Georges 1983; Miller 1989); reproductive potential in these species mayincrease with age.

High fecundity is matched by vulnerability of the egg and hatchling stage topredation. Parmenter (1985) estimated that in the absence of predation,survivorship of  Chelodina longicollis eggs at Laurendale near Armidale was

72%, but predation reduced this figure to 37%. Predation in the study areas wasestimated to be much higher, and close to 100%. Predators included theEuropean Red Fox, Vulpes vulpes and domestic cat, Felis catus. Water rats,goannas and crows exacted a high toll on nests of  Emydura macquarii in SouthAustralia, and foxes alone preyed on an estimated 93% of nests (Thompson1983). In contrast, nest predation is rare among Emydura sp. on Cooper Creek incentral Australia (Thompson 1983) where foxes are rare. Juvenile turtlesprobably fare much better in the water. Gibbons (1968) reported that the chanceof survival of hatchlings in the North American species, Chrysemys picta, wasvery high, although only 2% of the eggs survived to hatch. There are no usefuldata for post-hatching survival in Australian species.

There are few direct estimates of recruitment to freshwater turtle populations inAustralia, but it can be reasonably assumed that the populations are sustained bya trickle of new recruits, or by successful recruitment in the few years whenconditions allow many nests to survive to hatching. The longevity of adults isnecessary for populations to be sustained by such low or intermittentrecruitment.

The difficulties of studying growth in a long-lived slow growing vertebrate havediscouraged research on this topic in Australia, and few relevant studies havebeen published. In the temperate zones, freshwater turtles have an annual cycleof growth, even at latitudes that permit activity and feeding in all months(Georges 1982a). Growth of  Emydura krefftii (Fraser Island) is poorly correlatedwith body size, so it is not possible to calculate a satisfactory relationshipbetween age and size. In general, juveniles of all species so far studied growfaster than adults, and growth rate drops abruptly at the onset of maturity, whenavailable resources are redirected to reproduction (Georges 1985). Females of 

 Emydura and Chelodina species tend to grow faster than males and to reach

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larger maximum sizes (Parmenter 1976; Chessman 1978; Georges 1982a).Pseudemydura umbrina is the only species in which the males grow to largersizes than females (Burbidge 1981).

Onset of maturity typically occurs between seven and 12 years of age (Burbidge1981; Georges 1982a; Parmenter 1985). Females tend to mature at larger sizes

than males. There are no data on longevity for any Australian turtle, althoughGeorges (1982a) estimated that if a specimen of  Emydura krefftii (Fraser Island)grew at the fastest observed rate of any turtle in its size cohort, it would take atleast 69 years to reach maximum size.

There are relatively few estimates of population densities of freshwater turtles inAustralia. Chelodina longicollis is very abundant in the permanent dune lakes of the Jervis Bay Territory (up to 163.8 ± 34.2 turtles/ha; 9.9 ± 2.1 per 10 km of shoreline; Georges, Norris & Wensing 1986), and in lentic waters of the NewEngland Tableland (up to 400/ha in farm dams; Parmenter 1976). Densityestimates for Chelodina longicollis from Gippsland ranged from 160 turtles/hafor lagoons and 240 turtles/ha in farm dams, equivalent in both instances toabout 70 kg/ha (Chessman 1978). Population densities for Carettochelys

insculpta in the upper reaches of the South Alligator River have been estimatedat 33.8 ± 11.3 turtles/ha (equivalent to 67 turtles/km of channel or 227.4 kg/ha)(Georges & Kennett 1989). These estimates are high in comparison withestimates for species of freshwater turtles on other continents (Iverson 1982;Congdon, Greene & Gibbons 1986). However, in each of these studies, the turtledensities were probably inflated above the carrying capacity of the waters inwhich they were found because of seasonal contractions of their aquatic habitat(Georges & Kennett 1989; Kennett & Georges 1990).

An estimate of sustainable density is available for  Emydura krefftii on FraserIsland. In Lake Coomboo, they have a population density of 87 turtles/ha,equivalent to 28.8 ± 0.7 kg/ha. Biomass production was 18.5 ± 3.4 kg/ha

(Georges & Legler 1996).The population structure of freshwater turtle populations is highly variable,presumably dependent upon recent history of recruitment. Generally adultspredominate or are present in equal numbers to juveniles (Georges 1985), butage structures have not been determined for any Australian species. Sex ratiosare typically close to 1:1 for chelid turtles, which have genotypic sexdetermination, but skewed in favour of females for Carettochelys insculptawhich has environmental sex determination (Georges 1988a).

Most marine turtle eggs are laid on islands where potential egg predators arescarce. The principal threats to egg survival in Australia are early infertility and/ or embryonic death, flooding and erosion, and possibly microbial invasion

(Parmenter 1980; Limpus 1985; Limpus et al. 1983a, 1983d, 1983b). At mostrookeries, approximately 60 to 80% of eggs can be expected to producehatchlings which reach the beach surface. Exceptions are the eggs of Carettacaretta, which are subject to predation by Vulpes vulpes on the mainland northof Bundaberg, and those of  Natator depressus which are eaten by pigs (Sus sp.)and varanids on the mainland south of Bamaga and in the Northern Territory. Atmost rookeries, there is <2% loss of hatchlings to terrestrial predators during thebeach crossing (Limpus 1973), unless nocturnal predators are present (Limpuset al. 1983d).

The most intense predation on hatchlings probably occurs in the sea, howeverrecruitment and survivorship in feeding areas are poorly documented. Theprincipal predators of large marine turtles are tiger sharks, crocodiles, toothedwhales and man (Balazs 1980; Limpus et al. 1983d; Caldwell & Caldwell1969). Survivorship from egg to breeding adult is low, estimated at between afew per thousand and a few per ten thousand for C. caretta (Limpus 1985;Frazer 1986). Marine turtles in Australia display slow growth and are decades

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old at first breeding; for example >30 years for C. mydas, C. caretta and E. imbricata in the southern Great Barrier Reef (Limpus & Walter 1980; Limpus1985, 1992a). Because of the long period before sexual maturity, survivorship of large immature and adult marine turtles must be very high and the adult musthave a long breeding life to maintain population stability (Crouse, Crowder &Caswell 1987). Consequently marine turtle populations consist of a large

proportion of immature turtles (Limpus & Reed 1985a; Limpus 1985, 1992a).These same studies have identified widely varying sex ratios for C. mydas,C. caretta and E. imbricata inhabiting the southern Great Barrier Reef. Of thesespecies E. imbricata occurs at the lowest density (3.34 turtles/km2, and with alow biomass (0.82 kg/ha) (Limpus 1992a).

ECOLOGY

Diet

Turtles, with their rounded body form and heavy investment in bony skeletal

elements, are not renowned for their agility, and this has placed restrictions onthe foods they can exploit (Pritchard 1984). Most freshwater turtles areomnivorous, although some species have resorted to herbivory (for example,testudinids), and there are a few carnivorous specialists, such as Pseudemyduraumbrina, Chelus species from South America and Claudius species fromMexico. Carnivorous turtles may feed upon invertebrate prey even slower thanthemselves, while others rely upon stealth to secure more mobile prey.

Australian freshwater turtles typically are omnivorous (Fig. 17.3; Chapter 21).Temperate species of  Emydura, for example, consume a broad range of foodsincluding filamentous algae, periphyton (including sponges), a wide variety of aquatic macrophytes, aquatic macroinvertebrates, terrestrial insects which fall or

are blown onto the water, and carrion (Legler 1976; Georges 1982b; Chessman1986). There is a general tendency for juveniles of omnivorous species to bemore carnivorous than adults (Fig. 17.3; Georges 1982b; Moll & Legler 1971).

Omnivorous species of the wet-dry tropics, such as Carettochelys insculpta, Elseya dentata,  Emydura victoriae and  Emydura ‘australis’ , rely heavily uponthe seeds, fruits and leaves of riparian vegetation during the dry season,supplemented with aquatic macrophytes, algae, macroinvertebrates and carrion(Fig. 17.3; Legler 1976; Georges & Kennett 1989).

There is considerable overlap, especially among omnivorous species, in the dryseason habitat preferences and diets of the tropical turtles. It may be that thesesimilarities do not persist during the wet season, and that unique aspects of 

habitat and food preferences among species would emerge if studies wereconducted then. Alternatively, the highly variable nature of the tropical climate,and the interconnectedness of aquatic environments in wet seasons, may resultin interchange and re-invasion of river systems by ecologically similar species,at a greater rate than the slow process of competitive exclusion.

Newly hatched marine turtles presumably feed on the macroplanktonic algaeand/or animals in surface waters. Juvenile marine turtles, except  Dermochelyscoriacea, are principally benthic feeders during residence in shallow waters of the continental shelf. Dermochelys coriacea is a plankton feeder throughout life,feeding principally on jellyfish and planktonic tunicates (Brongersma 1972). InAustralian waters, C. mydas feeds mostly on seaweed, seagrass and mangrovefruits, C. caretta and Lepidochelys olivacea feed mostly on shellfish and crabs,

 N. depressus feeds mostly on soft corals and sea pens, and  E. imbricata eatssponges primarily. All species eat jellyfish and Portuguese man-of-war onoccasions (Lanyon, Limpus & Marsh 1989; Moody 1979; Guinea & Limpusunpub. data).

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Patterns of Habitat Use

Chelonians have specialised along two major paths with respect to habitatutilisation. Marine turtles of both the Cheloniidae and Dermochelyidae migrateas adults (Meylan 1982) and have a planktonic dispersal phase as young (Carr1986). Such highly mobile animals can utilise widely separated habitats,enabling them to optimise feeding and survival for different age/size classes andselect specialised breeding habitats. This is made possible by the relativestability and predictability of the marine ecosystem.

In freshwater and brackish water ecosystems the habitat is fragmented and aturtle will require wider physiological and morphological capabilities if it is todisperse widely as it will have to traverse terrestrial or marine habitats. Inreality, these capabilities are limited. As a result, freshwater and brackish-waterturtles tend to be restricted to particular river systems. Consequently, theirlifestyle needs to be more closely adapted to deal with the vagaries of localclimate and ecology.

Many species of Australian freshwater turtle occupy only permanent water of riverine and lentic ecosystems. Often several species will co-occur in the oneriver drainage. For example, six species of freshwater turtle occur in the Fitzroy-Dawson drainage of Queensland (Legler & Cann 1980). The dominance of each

Figure 17.3 Variation in the composition of the diet of Emydura krefftii with

increasing body size, as measured by the frequency of occurrence of prey instomachs by number or by volume. aty, atyid crustaceans; col, coleopteranlarvae; dip, dipteran larvae and pupae; eph, ephemeropteran nymphs; meg,megalopteran larvae; mis, miscellaneous aquatic insects; odo, odonatenymphs; par, parastacid crustaceans; pla, plant material; ter, terrestrialarthropods; tri, trichopteran larvae. [W. Mumford]

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in different microhabitats, as described in Chapter 21, together with variation indiet, presumably allow these species to co-exist in the same drainage. Indiscussing similar patterns of habitat fidelity on Fraser Island, Georges, Norris& Wensing (1986) speculated that the combined presence of  Emydura krefftiiand Chelodina expansa in the permanent dune lakes might virtually excludeC. longicollis because of competition for food. Such competition may be

exacerbated by the presence of fish (Chessman 1988).

Competition between species is remarkably difficult to demonstrate, primarilybecause the effects of passive competitive exclusion and character displacementare most evident at evolutionary time scales. Theories of passive competition forresources that explain the distribution and abundance of freshwater turtles inAustralia have very little direct experimental support and remain speculative.

Ephemeral aquatic environments are generally highly productive, particularly if they have dried completely before refilling. In addition, species that wouldotherwise compete with turtles for food, such as fish (Chessman 1984), are oftenunable to invade isolated ephemeral swamps. However, the numerous species of freshwater turtle that do exploit the benefits of such environments are still

subject to the problem of periodic and often unpredictable habitat loss. Manyspecies aestivate during such dry periods, including Pseudemydura umbrina,Chelodina rugosa, C. steindachneri and C. novaeguineae. Chelodina longicollismigrates to more permanent water.

In southern New South Wales, Chelodina longicollis migrates overland to seek refuge in permanent dune lakes when ephemeral swamps and ponds dry duringperiods of low rainfall. At such times, the number of turtles in the lakes reflectsthe carrying capacity of both the permanent and ephemeral waters of the region,and may well exceed the carrying capacity of the lakes alone. Exceptionallyhigh population densities were recorded for these lakes at the end of 1978–1983drought (Georges et al. 1986). As a result, turtles were in poor body condition,

grew slowly and failed to breed in 1986 (Kennett & Georges 1990).

Following a drought, Chelodina longicollis returns to the ephemeral waters asthey become available again, and act as the focus for reproduction, recruitmentand growth. Rain appears to stimulate a very rapid migratory response in this

Figure 17.4 Position and posture of the chelid Chelodina rugosa  duringaestivation [T. Wright]

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species. The migratory tendencies of  C. longicollis probably first evolved inresponse to selection for an ability to exploit productive ephemeral waters in theabsence of competition from fish (Chessman 1984) and other species of turtle(Georges et al. 1986; Chessman 1988). Once populations grew to approach thecarrying capacity of both the ephemeral and permanent waters of a region,individuals would have often found themselves occupying more restricted

permanent water during extended dry periods, where intense intraspecificcompetition for food would cause a sharp decline in growth and reproductiveoutput, as described above. In turn this would enhance selection for a rapidmigratory response and preference for ephermeral habitats.

Reproductive potential and onset of sexual maturity depend on size and not agein turtles (Gibbons 1982), so any delay in growth while occupying a droughtrefuge would have considerable consequences for reproduction of individuals.Under these circumstances, occupation of ephemeral waters would afford muchgreater selective advantages than could have been predicted from a comparisonof production in ephemeral and permanent waters alone.

In the tropics, Chelodina rugosa and C. novaeguineae attain highest densities inephemeral waters. They take advantage of high production during the wetseason and survive the dry season by migrating to permanent water or byaestivation. Chelodina rugosa occupies shallow ephemeral waterbodies of floodplains in low lying country adjacent to the Gulf of Carpentaria and betweenthe Arnhem Land and Kimberley plateaux. At the end of the dry season,C. rugosa buries beneath the mud of the waterbody in which it lives, to await thewet season innundation. It obtains its oxygen through a breathing hole until theground dries and cracks. This cycle of aestivation and activity of  C. rugosa isannual.

Chelodina novaeguineae occupies more marginal xeric environments whereannual rainfall is less predictable. When the ephemeral waterbodies dry,

individuals migrate to the surrounding scrub and aestivate beneath litter, in theburrows of other animals or in drainage crevasses. Aestivation byC. novaeguineae is not necessarily annual, and the species can aestivate for twoor more years (Cann pers. comm.).

Marine turtles utilise traditional nesting beaches in tropical and warm temperateregions and feed throughout the tropical and temperate seas of the world at somestage in their life cycles.  Dermochelys coriacea migrates to tropical nestingbeaches from open ocean feeding grounds as far afield as the high latitudewaters adjacent to pack ice (Pritchard 1971; Goff & Lien 1988). AlongAustralian shores,  D. coriacea nests infrequently in south-eastern Queensland,but it is commonly encountered feeding along the continental shelf to the south

of the Great Barrier Reef and off south-western Western Australia (Limpus &McLachlan 1979).

 Natator depressus nests only on continental islands and the mainland coast of Australia, largely avoiding beaches fringed by coral reef (Limpus, Gyurus &Miller 1988; Parmenter 1994). Unlike other marine turtles, post-hatchlingdispersal does not include an oceanic component and this species spends most of its life over the Australian continental shelf (Walker & Parmenter 1990). Thespecies is captured most frequently over soft bottom inshore habitats of theGreat Barrier Reef, within the Gulf of Carpentaria and throughout the ArafuraSea (Limpus et al. 1983c; Poiner & Harris 1996).

The remaining marine turtle species have an oceanic post-hatchling dispersalphase in the life cycle (Fig. 19.4; Carr 1986). In Australian waters, Cheloniamydas nests principally on islands near the oceanic margin of the continentalshelf, from the tropics northwards. Eretmochelys imbricata nests on islands onthe inner margin of the Continental shelf within the tropics, and  Lepidochelys

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olivacea nests principally on continental islands in north western Arnhem Land.Caretta caretta nests on the mainland and adjacent islands near the Tropic of Capricorn on both the east and west coasts (Limpus 1982, unpub. data).

The adult and immature turtles of these species occupy a diversity of feedingareas within a radius of some 2600 km of their rookeries (Limpus et al. 1992).Chelonia mydas feeds principally in coral reef and inshore seagrass pastures intropical and warm temperate areas (Lanyon et al. 1989). Eretmochelys imbricatais most commonly found feeding in coral and rocky reef habitats in tropicalareas (Limpus 1992a; unpub. data), while Lepidochelys olivacea feeds over softbottomed habitats across northern Australia (Harris 1994; Poiner & Harris1996). Caretta caretta feeds across a diversity of habitats including shallowcoral reefs to deeper soft bottomed habitats of the continental shelf along all butthe southern coast of Australia, although it may be most abundant in warmtemperate habitats (Limpus unpub. data).

BEHAVIOUR

Mating

Mating behaviour in turtles has been little studied. In freshwater species, matingbehaviour appears to vary between species (Murphy & Lamoreaux 1978),though cloacal touching precedes mounting in all species studied, as describedin Chapter 21. In marine species, the males and females aggregate for mating inthe vicinity of the nesting beach (Limpus 1993). The male mounts on top of thefemale, using all four flippers, and enlarged claws in cheloniid turtles, to graspthe carapace of the female. He then curls his elongate tail under the female tobring the cloacae together, allowing for insertion of the penis (Booth & Peters1972; Bustard 1972). Although a pair may be joined for many hours, copulationmay occur for a much shorter time.

Nesting

Typically, many freshwater turtles undertake nesting activity at night, triggeredby rain that falls during the breeding season (Goode 1967; Vestjens 1969). Watertemperatures appear important for other species, such as Carettochelysinsculpta, (Georges unpub. data) and a period of starvation followed by a flushof food availability is an important factor in the successful ovulation and nestingof Pseudemydura umbrina (Kuchling & DeJose 1989).

Nesting behaviour has been observed for a number of Australian chelids. Afterdigging a cavity, Emydura macquarii deposits its eggs, fills the excavation with

soil, and then drops its shell hard onto the ground to compact the soil in the fillednest (Goode 1965; see also Chapter 21). This tamping of the soil has beenobserved also in Chelodina longicollis (Vestjens 1969), Chelodina oblonga(Clay 1981), Chelodina expansa (Georges pers. obs.) and Pseudemyduraumbrina (Kuchling 1993).

The nesting behaviour of all marine turtle species is very similar (Bustard &Greenham 1969; Bustard, Greenham & Limpus 1971). The female searches outa nest site above the high tide level and excavates a depression (body pit) of variable depth, using all four flippers, and places her body down to the level of relatively firm sand. She then excavates a vertical, flask shaped egg chamberunder the tail using the hind flippers. The eggs are laid such that they dropdirectly into the egg chamber. At the completion of laying, moist sand isscooped into the chamber with the hind flippers, and tamped down at intervalsas the chamber fills. The tamping action of smaller species ( L. olivacea,

 E. imbricata,  N. depressus) may be quite vigorous. When the egg chamber isfilled, the female then digs her way forward for a variable distance, using her

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front flippers to dig sand from the front and fling it back, and the hind flippers topush sand to the middle back of the body pit. After the pit is refilled, the femalereturns directly to the sea. The duration of nesting varies from about an hour( L. olivacea,  E. imbricata,  N. depressus), to one and a half hours in C. carettaand D. coriacea, and three hours in C. mydas.

Following pipping, the hatchling remains for a day or so at the egg shell while ituncurls and internalises the remains of the yolksac (Miller 1985). The hatchlingsdig as a group to the surface over several days and emerge from the nest, usuallyat night (Bustard 1967b; Limpus 1973, 1985). On emergence, they orientate tolow elevation light horizons (Limpus 1971b; Salmon, Wyneken, Fritz & Lucas1992) which will normally direct them seawards. There is evidence of imprinting of the hatchlings to the earth’s magnetic field at the nest (Lohmann1991), and possibly to the water that they first contact (Grassman, Owens,McVey & Marquez 1984). Immediately the hatchlings reach the water theybegin swimming, and orientate perpendicular to wave fronts, a behaviour whichnormally takes them into deeper water (Salmon & Lohmann 1989). Thehatchlings at this stage live on yolk from the yolksac and do not feed or sleep

while on the beach or while swimming out to sea.

Thermoregulatory behaviour

Chelonians, like all reptiles, are ectothermic. They rely upon ambienttemperatures remaining within the range conducive to their general activity, orwhen this is not so, the animals use thermal sources and sinks within theirenvironment to maintain body temperatures within the preferred range. Reptileshave a vast repertoire of behaviours to regulate their body temperatures withinfairly narrow limits while active, quite irrespective of ambient temperatures, butturtles are limited in this regard because the water in which they live is thermallyconductive and they lack effective insulation. While large turtles can be

expected to have considerable thermal inertia, and circulatory mechanisms existto minimise heat transport from body core to the surface (Hutchison 1979), formost freshwater turtles, any elevation in body temperature achieved is quicklydissipated in the surrounding water.

Freshwater turtles thermoregulate primarily by seeking out warmer strata,typically at the water surface or in slower moving shallow waters, by basking inthe sun’s rays while floating at the surface, or by leaving the water altogether

Figure 17.5 Aquatic basking posture of the chelid, Emydura krefftii . (Afterphoto by J. Cann/ANT) [T. Wright]

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and basking (Fig. 17.6; Chapter 21). Basking is a major component of dailyactivity in Australian chelids (Webb 1978), and occurs in two major periods for

 Emydura sp. and Elseya latisternum. These shortnecked chelids bask until theyexhibit signs of discomfort similar to those reported in the PanamanianPseudemys (Moll & Legler 1971). These consist of tear secretion, gularmovements, panting, frothing from the mouth and wiping tears or water over thesurface of the head. Chelodina expansa and C. longicollis bask onlyoccasionally (Webb 1978).

Of the marine turtles, only Chelonia mydas leaves the water to bask and does somost frequently at breeding time (Whittow & Balazs 1982; Garnett & Crowley1985). Chelonia mydas, Caretta caretta, L. olicacea and N. depressus may bask by floating high at the water surface (Sapsford & van der Reit 1979; Limpusunpub. data). During sunny days basking can result in elevated bodytemperatures (Whittow & Balazs 1982; Sapsford & van der Reit 1979).However, since C. mydas will bask on beaches even at night when they wouldbe losing heat, it is doubtful if these basking activities are always primarily for

thermoregulation (Garnett & Crowley 1985; Limpus unpub. data).

ECONOMIC SIGNIFICANCE AND MANAGEMENT

Research

Research on freshwater turtles for management purposes requires informationon distribution and abundance, in order to assess the current population status of the species, to determine its specialisation for particular microhabitats, and toestablish a baseline for monitoring population trends. Hoop traps, baited withbread, sardines, meat, fish or a combination of these, are most useful forcapturing freshwater turtles (Legler 1960a). Hoop traps yield samples of 

 Emydura krefftii (Fraser Island) that are unbiased with respect to size and sex(Georges 1985).

Figure 17.6 Aerial basking posture in the chelid, Emydura macquarii .[T. Wright]

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Capture rates of turtles in baited hoop traps in the Northern Territory are verylow, with notable exceptions, and a modification of the design to enableaccumulation of turtles over a period of days or weeks has been developed byKennett (1992). Diving may be precluded by risk of attack by saltwatercrocodiles. Turtles can be collected in relatively clear waters using a dip-net andspotlight from a punt at night. This is particularly successful for capturing

Carettochelys insculpta,  Emydura victoriae and  Elseya dentata. Baited trapsreadily catch Chelodina longicollis, C. rugosa and all species of  Emydura and

 Elseya, but they are of limited use with C. expansa and Pseudemydura umbrina,species which feed principally or solely on live animals. Other methods of capture are discussed in Chapter 14.

Chelid turtles can be permanently marked by cutting notches in the marginalscutes and underlying peripheral bones with a file or small hacksaw. A semi-binary code is preferable to minimise the number of adjacent notches in any onequarter of the carapace. Notching is inappropriate for Carettochelys insculpta,which lacks scutes. Further, the carapace margin is well-vascularised, andcutting the peripheral plates, visible beneath the overlying skin, results in

unacceptable bleeding and such notches are not permanent (Georges & Kennett1988). Though cattle ear tags can be attached to the shell through a hole drilledin the lateral margins of the carapace, these are only useful for one to two years.A tagging system similar to that used on marine turtles (Limpus 1992b), orfreeze branding, are alternatives worthy of investigation for marking thisspecies.

Once a marked population is established, it is possible to estimate populationsizes using one of many mark-recapture techniques (Seber 1973). Catchabilityamong individuals varies considerably (Georges 1982a) and use of techniquescatering for such variation, such as the regression methods or frequency of capture methods (Seber 1973), should be explored as alternatives to thetraditional Peterson and Jolly-Seber methods, unless sampling intensity is high(>75% of the population marked).

Traditionally, reproductive biology of freshwater turtles has been studied bykilling turtles on a monthly sampling basis and examining the reproductiveorgans (Georges 1983). However, such destructive sampling is rapidlybecoming ethically unacceptable, and is not feasible for assessment of species atrisk. Radiography (Gibbons & Greene 1979), laparoscopy (Limpus & Reed1985a) and ultrasound scanning (Kuchling 1989) provide practical alternativesto dissection and direct examination of reproductive organs. Radiography can

Figure 17.7 Diagram showing themarking scheme used to identifyindividuals of Emydura krefftii .Notches are cut into the marginal

scutes and underlying peripheralbone with a file or small saw, to forma permanent and unique pattern.Individuals are identified by addingthe numbers assigned to particularscutes; for example turtle number 3is marked by notching scutes 1 and2 on the right rear of the carapace.

[D. Wahl]

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detect and allow counting of shelled oviductal eggs, but soft parts such ascorpora lutea or enlarged ovarian follicles are not revealed. Long-term effects of repeated exposure of turtles to X-rays is not known.

Gonadal structure can be examined, measured and biopsied by laparoscopy, butthere are limitations on accurate counting and some measurements (Limpus1992a). Ultrasound scanning is an accurate method to measure and count largestructures such as eggs and follicles. However, there are limitations on the depthof reliable viewing with large animals, and small structures, such as corporaalbicantia, may not be resolved in the images produced. Though these newtechnologies have their strengths and weaknesses, they have the potential tocontribute significantly to population studies.

Stomach contents can be obtained by stomach flushing (Legler 1977). A 12Vsubmersible pump is used to supply a steady flow of water, which is passed intothe stomach through a flexible plastic tube (Georges et al. 1986). The water thenpasses back up the oesophagus carrying food particles with it. Water flow isadjusted for turtles of different sizes by interchanging tubes of differentdiameters. The food particles are caught in gauze and transferred to 70% ethanol

for later sorting and identification.

Percentage composition of the diet by number, weight or volume, andoccurrence (Windel & Bowen 1978) are traditionally used to assess the relativeimportance of various food types in the diet. Caution must be exercised in theinterpretation of diet from stomach contents, as the proportional representationof foods in the stomach does not reflect diet in a strictly quantitative sense.Different foods differ in their digestibility and rate of passage through the gut.Nor do animals forage at random, so foods found in the stomach of an individualat one time do not necessarily provide a random selection of foods eaten by thatindividual over time. To assess the degree of dietary specialisation, one mustcompare the foods eaten with the foods potentially available in the environment,either qualitatively or quantitatively (Georges et al. 1986).

Criteria for Setting Conservation Priorities

Australian freshwater turtles do not often fall into the category of species inconflict with community goals, values and aspirations. Nor are they a majoreconomic resource, at least for Australian communities. Priorities for theirconservation must therefore be based on values that cannot be measured on aneconomic scale.

There are three intrinsic attributes with a bearing on the conservation priorityaccorded a species. The first is distinctiveness, important because it mayenhance both its aesthetic and scientific value. If a species is taxonomically

distinct, then it will lack close relatives which may share some or many of itsfeatures, and much may be lost if such a species becomes extinct. The secondattribute is rarity. Rarity in the colloquial sense may increase the aesthetic valueof a species, and so increase public pressure for adequate conservationmeasures. Rarity in the biological sense combines both abundances anddistribution (Rabinowitz, Cairns & Dillon 1986). An abundant species with ageographically restricted distribution can be quite vulnerable to extinctionthrough habitat destruction, climatic change or disease, whereas an ubiquitousspecies with low abundances throughout its range may be quite secure. The thirdattribute is a species’ intrinsic vulnerability to decline and eventual extinction.Rarity, low fecundity, susceptibility to disease, specialisation on particular foodsor other environmental attributes are all examples of factors contributing to a

species’ vulnerability to extinction.

Extrinsic factors also have a role to play in setting conservation priorities. Arethere current threats to populations, and is there a pressing need for interventionto negate or offset the deleterious effects of human activities or natural events?

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Opposing factors include logistic constraints—whether they be legal, economic,social or biological— that diminish the feasibility of conservation measures orrender them impractical. Feasibility must be gauged against the objectives of management, namely to preserve the current Australian turtle diversity bypreventing further extinctions. A second objective, and one that requires moreinvestment to achieve, is to conserve species in the context in which they

evolved so that we also preserve the processes that maintain genetic diversityand lead ultimately to speciation.

Conservation Priorities

Among freshwater forms, Pseudemydura umbrina sits high on the list of priorities. It is distinctive both taxonomically and morphologically. It representsa monotypic genus, and its nearest living relatives may be among the chelids of South America (Legler 1981). The species is exceptionally rare (Burbidge 1981;Kuchling & DeJose 1989), being restricted to a wild population of less than 30individuals near Perth in Western Australia. It is vulnerable by its inflexiblepreference for ephemeral waters, even when permanent water is available and

accessible (Burbidge 1983), in a climatic regime where its habitat is flooded inthe colder months of the year. It also has the lowest fecundity of any Australianchelid, laying only one clutch of three to five eggs per season. The species isthreatened by habitat loss through past draining of swamps for land reclamationand by predation on eggs and adults by introduced foxes, which are devastatingpredators of freshwater turtles throughout their range (Parmenter 1976;Thompson 1983; Palmer-Allen et al. 1991). Current measures are directed atcaptive breeding and re-introduction to an area within its former range(Burbidge, Kuchling, Fuller, Graham & Miller 1990; Kuchling & Bradshaw1993; Kuchling & DeJose 1989; Kuchling, DeJose, Burbidge & Bradshaw1992).

Carettochelys insculpta is Australia’s most distinctive turtle, bothmorphologically and taxonomically. It is the sole remaining species in its family,and as such represents all that remains of 40 million years of evolutionindependent of any other extant lineage (Chen, Mao & Ling 1980). The specieswas once considered to be one of the rarest turtles in the world (Groombridge1982), but it now appears that where it is found, it may be locally quite abundant(Georges & Kennett 1989). It is rare in the sense of being geographicallyrestricted, as a family, to southern New Guinea and northern Australia. InAustralia, it is distributed from the Victoria River in the west to the GoomadeerRiver in the east, but abundant only in the Daly and Alligator drainages. Thespecies is vulnerable by virtue of its limited distribution, and becausestereotyped nesting habits render it susceptible to over-exploitation. Adults and

their eggs are highly regarded as food by indigenous peoples throughout itsrange, and exploitation for food is thought to be threatening populations in NewGuinea (Groombridge 1982). Habitat degradation is the major potential threat toAustralian populations (Georges 1988b).

Third on our list of priorities for freshwater turtles is  Rheodytes leukops. It isdistinctive enough to be placed in a monotypic genus, and has no clear affinitieswith any other Australian freshwater turtle (Georges & Adams 1992).Morphologically, it is quite striking because of its capacity to extract oxygenfrom water using well-vascularised gills in a well-ventilated cloaca (Chapter16). The species is restricted to a single drainage, the Fitzroy-Dawson system of central Queensland, and while there are no data on abundances, it is consideredquite rare by virtue of its limited distribution. The species is vulnerable as aresult, and because it is specialised for life in the riffle—fast flowing brokenwater. With increasing numbers of dams and weirs, riffle is an endangeredmicrohabitat. The biology of  Rheodytes leukops is poorly known, and itsconservation priority may change with further study.

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Other distinctive species are largely ignored during considerations onconservation, because they are so poorly studied. Many are undescribed. Thethree undescribed species of  Elseya in New South Wales, the new genuscolloquially known as shortnecked alpha, the Arnhem Land Chelodina are allrestricted in distribution, but nothing is known of their biology or populationstatus. Emydura subglobosa of the Jardine River on Cape York, is rare, but not

considered a high priority because it is not taxonomically distinct above thespecies level and it is common in New Guinea.

Rare species are not the only ones to attract the attention of conservationbiologists. Recent biochemical studies (see Georges & Adams 1992) haveshown E. macquarii from the Murray-Darling drainage, E. krefftii from coastalQueensland and E. signata from coastal New South Wales to be a single species,sharing even rare alleles. However, morphological and genetic variation suggestthat they are in the process of allopatric speciation. This speciation processcould be brought to an abrupt halt if exchange of specimens between drainagesis artificially enhanced, say by distribution of hatchling  Emydura by the pettrade. In contrast, specimens of  Chelodina longicollis frequently migrate

overland and may easily migrate from drainage to drainage. Genetic andmorphological variation throughout its range is slight, and commercialdistribution of hatchlings and subadults would have little impact on geneticprocesses leading to speciation. These are important considerations if we are toaddress seriously the second conservation objective listed above.

Among the marine turtles, C. caretta populations that nest in eastern Australia,have declined by more than 50% in the past decade. This decline has beenattributed mostly to accidental drowning in fishing gear in Australian waters(Limpus & Reimer 1994). There is wide spread and large-scale hunting of C. mydas and E. imbricata populations that nest in northern Australia. There isno evidence to suggest that the populations are large enough to support the

current harvests in the long term. The small number of  L. olivacea

which drownin fishing gear annually in Australia may be excessive for the small nestingpopulations that occur in Australia. For these species at least, there must beconcern for the long term viability of the species in Australia and activeconservation management of their populations is required both in Australia andin neighbouring countries.

Management must address a wide range of problems which may be grouped intothree broad areas: fishing, development and predation by feral animals.Traditional and commercial harvests of turtles are often carried out in remoteareas where enforcement is difficult; fishermen are often reluctant to modifyfishing gear in order to reduce turtle capture and death. Development of coastalareas for tourism, agriculture and real estate have negative impacts on turtle

nesting and feeding habitat. Predation of eggs and hatchlings by feral foxes andpigs in Queensland may result in future declines in C. caretta and N. depressuspopulations.